FIG. 1 shows a conventional semiconductor circuit as disclosed for example in JP-A-62-23629. In the figure, reference numeral 1 denotes an input terminal which inputs a high frequency signal, 2 is a bias control circuit which supplies a bias current to a variable damper 3 when a signal level of a high frequency signal is higher than a reference level, 3 is a variable damper which damps a high frequency signal when a bias current is received from the bias control circuit 2, 4 is an amplifier which amplifies a high frequency signal damped by the variable damper 3 and 5 is an output terminal which outputs a high frequency signal amplified by the amplifier 4.
The operation of the invention will be outlined below.
Firstly, when a semiconductor circuit is used for signal reception, reception characteristics can deteriorate as a result of the variable damper 3 damping high frequency signals when the signal level of the high frequency signal input from the input terminal 1 is low.
On the other hand, if however damping of the high frequency signal by the variable damper 3 is terminated, the amplifier 4 may be saturated as the signal level of the high frequency signal is high.
In the above conventional example, when a high frequency signal is input from the input terminal 1, the bias control circuit 2 compares the signal level of the high frequency signal with a reference level. When the signal level of the high frequency signal is lower than the reference level, supply of the bias current to the variable damper 3 is terminated and the attenuation of the variable damper 3 is set to 0 dB.
In this way, reception characteristics may be improved by amplifying the signal level of the high frequency signal since a high frequency signal is output to the amplifier 4 without being damped by the damper 3.
Furthermore the bias control circuit 2 supplies a bias current to the variable damper 3 when the signal level of the high frequency signal is higher than the reference level and the attenuation of the variable damper 3 is set to XdB.
In this way, saturation of the amplifier 4 may be avoided because a high frequency signal is output to the amplifier 4 after the signal level of the high frequency signal is damped by the variable damper 3 with an attenuation of XdB. However in this case, although the S/N ratio by which the variable damper 4 damps the high frequency signal deteriorates to some degree, a S/N ratio required for reception is maintained since the signal level is high enough to saturate the amplifier 4.
When the attenuation of the variable damper 3 is set to 0 dB, as stated above, the variable damper 3 does not damp high frequency signals and a high frequency signal is output to the amplifier 4. When a high frequency signal is output to the variable amplifier 3, reception characteristics deteriorate and the amplifier 4 increases reception noise due to a certain degree of insertion loss.
In such a case, as shown in FIG. 2, it is possible to eliminate deterioration of reception characteristics by disposing the variable damper 3 on the output side of the amplifier 4. However such an arrangement will not allow saturation of the amplifier 4 to be avoided.
Since a conventional semiconductor circuit is constructed as above, when the variable damper 3 is disposed on the input side of the amplifier 4, it is possible to prevent saturation of the amplifier 4. However when the signal level of the high frequency signal is lower than a reference level, reception characteristics deteriorate due to insertion loss. When the variable damper 3 is disposed on the output side of the amplifier 4, it is possible to prevent deterioration of reception characteristics. However in this case, saturation of the amplifier 4 can not be avoided.
The present invention is proposed to solve the above problems and has the object of providing a semiconductor circuit which can prevent deterioration of reception characteristics and which can prevent saturation of an amplifier.